Urination

Anatomy

bladder schematic

The bladder serves to store urine until an appropriate time for urination. The wall of the bladder consists of the detrusor muscle, which is smooth muscle, and an overlying mucosal layer which is lined by uroepithelium (also known as transitional epithelium).

Urine is delivered to the bladder from the kidneys via the ureters, which enter the posterior bladder wall obliquely, with their openings at the trigone. The trigone is a bit of thickened smooth muscle on the posterior wall of the bladder. The oblique orientation of the ureter entry into the bladder ensures that the ureters will squeeze shut when the detrusor muscle contracts during urination.  This prevents reflux of urine back towards the kidneys.

The outlet of the bladder is the urethra. In females, the urethra is quite short, while in males, the urethra is longer and divided into 3 segments (the prostatic urethra, the membranous urethra, and the penile urethra).  There is a thickening of the smooth muscle at the bladder neck which functions as the internal urethral sphincter.  The external urethral sphincter is skeletal muscle located in the pelvic floor.


Innervation

bladder innervation

The bladder is controlled by the central nervous system.  The figure at left depicts the innervation of the different muscles in the bladder.  The smooth muscles are controlled by the two different divisions of the autonomic nervous system.  Parasympathetic inputs stimulate contraction of the detrusor muscle.  Sympathetic inputs stimulate contraction of the internal urethral sphincter.  Somatic efferent neurons stimulate contraction of the external urethral sphincter, which is skeletal muscle.

The walls of the bladder stretch as the bladder expands during bladder filling. This is sensed by bladder afferent neurons whose sensory dendrites are located in the bladder wall. Bladder afferents project to the spinal cord and to various regions in the brain that are responsible for coordinating the efferent output to the bladder and urethra.



parasympathetic sympathetic somatic efferent

urine storage
(during bladder filling)
inhibited stimulated stimulated

urination

stimulated inhibited inhibited

There are two modes of operation for the bladder, which are outlined in the table above: either urine storage, or urination. During the time that the bladder is filling, when urine is stored in the bladder, the parasympathetic input to the detrusor muscle is inhbited, and there is activation of the sympathetic and somatic efferent pathways, causing the two urethral sphincters to contract. The afferent information from the bladder provides a sense of bladder fullness and the urge to urinate. At an appropriate time and place for urination, the parasympathetic efferents become active, causing contraction of the detrusor muscle. At the same time, there is coordinated inhibition of the sympathetic efferents and the somatic efferents so that the internal and external urethral sphincters relax.



Quick Quiz


Fill in Answer Correct False Correct Answer
1. What is the name of the smooth muscle that makes up the bladder wall?
2. What type of muscle forms the internal urethral sphincter?
3. What type of muscle forms the external urethral sphincter?
4. What part of the bladder or urethra is innervated by sympathetic neurons?
5. When are parasympathetic inputs to the bladder inhibited?


(Spelling must be correct)